Mice were exposed to approximately 20 ppm of nitrogen dioxide (NO2) for 5-6 hr, or 1 g/kg of body weight of morpholine by gavage, or both. Treatments were repeated daily for 5 consecutive days. N-nitrosomorpholine (NMOR) was found in whole carcasses (16 to 146 ng/mouse) in all animals that had been exposed to both NO2 and to morpholine, but not in tissues from animals that had been exposed to either chemical alone. Approximately one-third of the NMOR was found in the gastrointestinal tract. The coadministration of 2 g/kg of sodium ascorbate or 1 g/kg of alpha-tocopheryl acetate had no effect on the amount of NMOR that was found in any tissue. Approximately one-third of the total amount of NMOR that was found in the body was found in the stomach. Another possible product of the interaction of NO2 and morpholine, N-nitromorpholine, was not detected in any tissue. We concluded that the repeated, concurrent exposure of mice to NO2 by inhalation and to morpholine by gavage resulted in the in vivo formation of significant quantities of NMOR. The biological significance of the observation remains unknown.